This invention relates generally to a miniature motor used in audio equipment, precision instruments, automotive electrical equipment, etc. and more specifically to a miniature motor having a positive temperature coefficient resistor for controlling overcurrent flowing in a motor armature by detecting the temperature of the motor.
FIG. 1 is a longitudinal sectional front view of the essential part of a conventional type of miniature motor. In FIG. 1, numeral 1 denotes a case made of a metallic material, such as mild steel, formed into a bottomed hollow tubular shape and having a permanent magnet 2 formed into an arc-segment shape, for example, and fixedly fitted to the inner circumferential surface thereof. In the case 1 provided is a rotor 5 comprising an armature 3 facing the permanent magnet 2 and a commutator 4.
Numeral 6 denotes a case cap made of an insulating material, such as a resin material, and fitted to an open end of the case 1. Numeral 7 denotes a brush having a sliding contact shoe at the free end thereof for making sliding contact with the commutator 4, and provided in the case cap 6, together with an input terminal 8 electrically connected to the brush 7. Numerals 9 and 10 denote bearings, fixedly fitted to the bottom of the case 1 and the central part of the case cap 6, respectively, for supporting the shafts 11 and 12 constituting the rotor 5.
With the aforementioned construction, when power is fed from the input terminals 8 and 8 to the armature 3 via the brushes 7 and 7, and the commutator 4 constituting the rotor 5, rotating force is imparted to the armature 3 placed in a magnetic field formed by the permanent magnet 2 fixedly fitted to the inner circumferential surface of the case 1, causing external equipment (not shown) to be driven via the output-side shaft 11.
The miniature motor as shown in FIG. 1 has a wide range of applications, as described earlier, and particularly useful as motors for driving automotive electrical equipment, such as electrically operated rear-view mirrors, electric-powered door-window regulators, etc. Having low power, however, miniature motors could readily be overloaded (stalled in extreme cases) when a trifling trouble (such as rusting, entry of foreign matter, etc. ) occurs in the driven part thereof, or when the driven part reaches its operating limit. This leads to unwanted overheating, or burning of the windings of the armature 3.
As a measure to solve the aforementioned drawbacks, means for feeding power to the armature 3 via a positive temperature coefficient resistor has heretofore been used. FIG. 2 is a partially exploded perspective view illustrating an example of miniature motor having a positive temperature coefficient resistor, and FIGS. 3 and 4 are enlarged cross-sectional views of the essential part illustrating the state where component members in FIG. 2 are assembled: both being the prior art proposed by the present applicant et al. (refer to Japanese Published Unexamined Utility Model Application No. Hei-2(1990)-41664).
In FIGS. 2 through 4, numeral 22 denotes an input terminal fixedly fitted to the case cap 6. Numeral 23 refers to a positive temperature coefficient resistor formed into a quadrilateral strip shape, for example, with both sides thereof coated with electrodes (not shown). Numeral 24 denotes a brush base having a power feeding brush piece 7a provided at the free end thereof and connected to the brush 7 integrally or in such a manner as to ensure electrical continuity.
Grooves 22a, 23a and 24a having contours corresponding to the cross-sectional shape and dimensions of the input terminal 22, the positive temperature coefficient resistor 23 and the brush base 24, respectively, are provided on the case cap 6 in a mutually adjoining and almost parallel manner. The groove 22a are provided by passing through the case cap 6, while the other grooves 23a and 24a are provided so that they are opened to the inner side surface of the case cap 6.
Lug pieces 22b and 24b are provided on the input terminal 22 and the brush base 24 at the respective positions thereof opposing to the positive temperature coefficient resistor 23. The input terminal 22, the positive temperature coefficient resistor 23 and the brush base 24 of the aforementioned construction are fixedly fitted by inserting them into the grooves 22a, 23a and 24a provided on the case cap 6 in directions as shown by arrows A.sub.I, A.sub.2 and A.sub.3, respectively. Though not shown in the figures, the other input terminal and brush base are also fixedly fitted in the same manner so as to form their respective pairs, together with the input terminal 22 and the brush base 24.
By assembling the components in the aforementioned manner, the positive temperature coefficient resistor 23 is held by the lug pieces 22b and 24b provided on the input terminal 22 and the brush base 24, and performs predetermined functions, as shown in FIG. 3. Measures for preventing overheat may not be required, depending on the mode of use of the miniature motor. In such a case, the lug pieces 22b and 24b, which have sufficient resiliency and have been preloaded to a considerably large degree, can be brought into direct contact with each other, thereby ensuring electrical continuity merely by extracting the positive temperature coefficient resistor 23, as shown in FIG. 4.
FIG. 5 is a longitudinal sectional front view illustrating the essential part of another embodiment of a miniature motor of a conventional type. Like parts are indicated by like numerals shown in FIG. 1. In FIG. 5, numeral 16 denotes a brush formed into a hollow quadrangular prism shape having a square or rectangular cross-section, for example, and slidably fitted to a brush holder 15 provided on the inside end face of the case cap 6 so as to make sliding contact with the commutator 4.
Numeral 13 denotes a spring for forcing the brush 16 onto the commutator 4. Numeral 8 denotes an input terminal passed through and fixedly fitted to the case cap 6, and electrically connected to the brush 16 via a pig-tail wire 14 on the inside end of the case cap 6.
With the aforementioned construction, when electric current is fed from the input terminals 8 and 8 to the armature 3 via the pig-tail wires 14 and 14, the brushes 16 and 16, and the commutator 4 constituting the rotor 5, rotating force is imparted to the armature 3 placed in a magnetic field formed by the permanent magnet 2 fixedly fitted to the inner circumferential surface of the case 1, causing the rotor 5 to rotate and external equipment (not shown) to be driven via the output-side shaft 11.
Even in the miniature motor shown in FIG. 5, means of providing a positive temperature coefficient resistor is adopted as a measure for preventing overheat. FIG. 6 is an enlarged cross-sectional view of the essential part of the input terminal shown in FIG. 5 and the vicinity thereof, corresponding to FIG. 3 above. Like parts are indicated by like numerals shown in FIG. 3.
In FIG. 6, numeral 17 denotes a connecting piece having a lug piece 17b provided at a position facing the positive temperature coefficient resistor 23, and fixedly fitted into a groove 17a provided on the case cap 6. The lower end of the connecting piece 17 is connected to the pig-tail wire 14 for electrically connecting to the brush 16 shown in FIG. 5. With the aforementioned construction, the positive temperature coefficient resistor 23 is held by lug pieces 22b and 17b provided on the input terminal 22 and the connecting piece 17, and performs the predetermined function, in the same manner as shown in FIG. 3.
In the miniature motor having the aforementioned construction, the positive temperature coefficient resistor 23 is held by the lug pieces 22b and 24b or 17b provided on the input terminal 22, the brush base 24 or the connecting piece 17. In this construction, however, the positive temperature coefficient resistor 23 may fall from the case cap 6 during service due to vibration or other external forces because the holding force is given only by frictional force.
To increase the holding force, the resiliency of the lug pieces 22b, 24b and 17b can be increased, but too large a resiliency would increase the pushing force required to insert the positive temperature coefficient resistor 23 between the lug pieces 22b and 24b or 17b. This not only makes the insertion of the positive temperature coefficient resistor 23 troublesome, but may cause damage to the electrode provided on the surface of the positive temperature coefficient resistor 23, leading to the loss of the functions of the positive temperature coefficient resistor 23 in extreme cases.
Although the input terminal 22, the brush base 24 and the connecting piece 17 are generally formed into a thickness of about 0.8 mm, for example, the range of controlling the resiliency imparted to the lug pieces 22b, 24b and 17b is limited due to the small size of these components. This makes it extremely difficult to set the proper hold force of the positive temperature coefficient resistor 23.
If the miniature motor of the aforementioned construction is of a specification where the positive temperature coefficient resistor 23 is omitted, the lug pieces 22b and 24b or 17b are brought into direct contact with each other, as described above. If the miniature motor of such a construction is of a specification requiring a positive temperature coefficient resistor 23, the accidental omission of the positive temperature coefficient resistor 23 during assembling operation may be overlooked in the final inspection because the lug pieces 22b and the 24b or 17b are brought into direct contact with each other, forming electrical continuity. Thus, there is a danger where a miniature motor from which the positive temperature coefficient resistor 23 is accidentally omitted may be judged as acceptable in the final inspection.